Organic electroluminescent devices are known which employ an organic material for light emission. For example, WO 90/13148 describes such a device comprising a semi-conductor layer comprising a polymer film which comprises at least one conjugated polymer situated between electrodes. The polymer film in this case comprises a poly(para-phenylene vinylene) (PPV) film which is capable of light emission when electrons and holes are injected therein. Other polymer layers capable of transporting holes or transporting electrons to the emissive layer may be incorporated into such devices.
For organic semiconductors important characteristics are the binding energies, measured with respect to the vacuum level of the electronic energy levels, particularly the “highest occupied molecular orbital” (HOMO) and “lowest unoccupied molecular orbital” (LUMO) levels. These can be estimated from measurements of photoemission and particularly measurements of the electrochemical potentials for oxidation and reduction. It is well understood in the field that such energies are affected by a number of factors, such as the local environment near an interface, and the point on the curve (peak) from which the value is determined. Accordingly, the use of such values is indicative rather than quantitative.
FIG. 1 shows a cross section of a typical device for emitting light. FIG. 2 shows the energy levels across the device. The anode 1 is a layer of transparent indium-tin oxide (ITO) with a work function of 4.8 electron volts. The cathode 2 is a LiAl layer with a work function of 2.4 electron volts. Between the electrodes is a light emissive layer 3 of PPV, having a LUMO energy level 5 at around 2.7 electron volts and a HOMO energy level 6 at around 5.2 electron volts. Holes and electrons that are injected into the device recombine radiatively in the PPV layer. An important feature of the device is the hole transport layer 4 of polyethylene dioxythiophene (PEDOT). This polymer is disclosed in EP 0686662. This provides an energy level at about 5.2 electron volts which helps the holes injected from the ITO to reach the HOMO level in the PPV.
It should be noted here that values stated for energy levels, work functions etc. are generally illustrative rather than absolute. For example, the work function of ITO can vary widely. It is well known that the value can depend on ITO deposition process and history.
Known device structures also may have an electron transport layer situated between the cathode 2 and the light emissive layer 3. This provides an energy level which helps the electrons injected from the cathode to reach the LUMO level of the material constituting the light emissive layer. Suitably, the electron transporting layer has a LUMO energy level between the LUMO energy levels of the cathode and the light emissive layer or matched to the LUMO energy level of the light emissive layer.
One disadvantage associated with multiple layered devices is that where the layers are deposited from solution it is difficult to avoid one layer being disrupted when the next is deposited, and problems can arise with voids or material trapped between the increased number of inter-layer boundaries. Appl.Phys.Lett. 51, 913-915 (1987) is concerned with organic thin-film electroluminescence. Devices disclosed in this document consist of a hole-transporting layer of an aromatic diamine and an emissive layer of 8-hydroxyquinoline aluminium. ITO is used as the hole-injecting electrode and a magnesium-silver alloy as the electron-injecting electrode.
As is disclosed in Nature, 397, 121-128 (1999) TPD is used as a hole transport layer. However, this molecular material has the disadvantages associated with using small molecule layers in a device. Similarly, this document discloses that PBD is known as an electron transport layer. Again, this has the disadvantageous device characteristics associated with using small molecule layers as compared with polymer layers in electroluminescent devices.
The use of polymers in general in light emitting devices, and particularly as charge transport materials is very attractive. Polymers show excellent device characteristics. These device characteristics include good efficiency, processability and device lifetime.
Poly(arylamines) are disclosed in U.S. Pat. No. 5,728,801 as useful charge transport layers in light-emitting diodes. This document further discloses that triarylamines are used as charge transport materials, specifically positive charge transport materials, because they are easily oxidised to the corresponding radical cation. The usefulness of the possibility of using these polymers in film form is discussed in this document.
In view of the above, there still remains a need to simplify the structure of light emitting devices, thus, simplifying manufacturing processes and reducing production costs.